19

CHAPTER - 2

SYNTHESIS, GROWTH AND CHARACTERIZATION OF ACETOPHENONE THIOSEMICARBAZONE AND BENZALDEHYDE

THIOSEMICARBAZONE MONOHYDRATE SINGLE CRYSTALS

2.1 INTRODUCTION

Progress in the area of non-linear optics depends upon the development

of new materials. When compared with the inorganic materials, organic and

semiorganic materials have been attracting a great deal of attention, as they

have large optical susceptibilities, inherent ultra fast response time and good

optical properties (Dhanuskodi and Mary 2003, Prasad and Williams 1991,

Vijayan et al. 2003a, 2003b). Organic molecules containing electron

conjugation systems asymmetrized by the electron donor and acceptor groups

are highly polarizable entities for nonlinear optical (NLO) applications (Tansuri

et al. 2004). The donor/acceptor groups of benzene derivatives can produce

high molecular nonlinearity. Recently there has been considerable interest in

the co-ordination chemistry of aryl hydrazones such as semicarbazones,

thiosemicarbazones and guanyl hydrazones because of their importance for

drug design (Beraldo and Gambino 2004) organocatalysis and for the

preparation of hetero cyclic rings (Bondock et al. 2007). Further extensive

electron delocalization reported in these types of structures helps the

thiosemicarbazone complexes to acquire second harmonic generation (SHG)

efficiency (Domiano et al. 1969, Andreetti et al. 1970, Muharrem and Namik

2005a, 2005b). Single crystals of acetophenone thiosemicarbazone (APTSC) and

benzaldehyde thiosemicarbazone (BTSC) are the potential organic nonlinear

optical materials, which belongs to the ketone and aldehyde group of

compounds respectively.

Three dimensional crystal structure of APTSC was determined by single

crystal X-ray diffraction study. APTSC crystallizes in the orthorhombic space

group Pbca (Santhakumari et al. 2010). The cell parameter values are

a=15.429(3) Å, b=7.127(7) Å, c=8.340(7) Å. Crystal structure of APTSC is

stabilized by N―H…S interamolecular hydrogen bonds and the molecule is

nearly planer. Sheng-Jiu Gu and Kai-Mei Zhu (2008) have reported on the three

dimensional crystal and molecular structure of benzaldehyde thiosemicarbazone

20

(BTSC) monohydrate. The cell parameter values are a=6.1685(10) Å,

b=7.6733(12)Å, c=21.131(2)Å. The crystal belongs to the well known

noncentrosymmetric orthorhompic system with space group P212121.

To the complete comprehensive knowledge of authors there is no report

available on the growth and characterization of these compounds. Hence, the

results of synthesis, growth, crystal structure and characterizations of APTSC

and BTSC are presented in section A and section B respectively this chapter.

Section A

Growth and characterization of acetophenone thiosemicarbazone

(APTSC) single crystals

2.2 Experimental

2.2.1 Synthesis, solubility and growth of APTSC

APTSC was synthesized by reacting analytical grade acetophenone

(C8H8O) and thiosemicarbazide (CH5N3S) in 1:1 molar ratio in the distilled

water. The prepared solution was stirred well using magnetic stirrer and

if the solution appeared to be turbid, methanol/ethanol was added

further and gently warmed until a clear solution was obtained. The

APTSC compound was synthesized according to the chemical reaction

depicted in the scheme.

Scheme. The reaction mechanism of APTSC

21

Studies on the solubility at various temperatures showed that APTSC

has relatively high solubility in ethanol compared to that of in methanol

(Fig. 2.1). Hence ethanol was chosen as the solvent in the present work.

Fig. 2.1 Solubility curve of APTSC Transparent APTSC crystals of about 0.5 x 0.5 x 0.5 mm3 were

obtained from ethanol by slow evaporation at room temperature and one

of the optically good quality crystals was used as seed crystal. Slow

evaporation at room temperature yielded a good quality single crystal of

dimensions 8 x 7 x 2 mm3 in a growth period of 15 days and is shown in

Fig. 2.2.

Fig. 2.2 The as-grown APTSC crystal

22

2.3 FTIR and laser Raman spectral analyses of APTSC

FTIR and Laser Raman spectral analyses were carried out to

characterize the functional groups of the APTSC crystal molecules. FTIR

spectrum was recorded for the purified sample using Perkin Elmer

Paragon-500 by KBr pellet technique between 400 cm-1 and 4000 cm-1

and is shown in Fig. 2.3. The band obtained at 1589 cm-1 is due to the

formation of imine group (C═N) between ketone and amine (Silverstein

and Webster 1998). A weak absorption, observed at 2958 cm-1 is due to

C―H stretching. The peak at 3402 cm-1 is due to NH2 asymmetric

stretching and an absorption peak at about 3140 cm-1 is due to NH

stretching. The band appeared at 1497 cm-1 confirms the C═C stretching

frequency. As mono substituted benzenes show C―H deformation

vibration in the region 710-690 cm-1 (Sharma 2000), the presence of C―H

deformation is evident from the vibration at 685 cm-1.

Fig. 2.3 FTIR spectrum of APTSC crystal

23

The peak observed at 841 cm-1 is due to the out-of-plane aromatic

C―H bond. The C═S stretch of the thiosemicarbazide moiety is observerd

at 1093 cm-1. Absence of characteristic ketone band between 1725-1705

cm-1 (C═O stretching) indicates that there is no ketone group in the final

product (Kemp 1991). The laser Raman spectrum (Fig. 2.4) was recorded

using Raman System, R-3000, solid state laser (532 nm with green light)

in the range of 100-3600 cm-1. In laser Raman spectrum C―H stretching

and C―H deformation (out of plane) vibrations are observed at 2795 cm-1

and 857 cm-1 respectively.

Fig. 2.4 Laser Raman spectrum of APTSC crystal

24

The C═N and C═C stretching vibrations are observed at 1606 cm-1

and 1504 cm-1 respectively. NH2 rocking is observed at 1113 cm-1. The

observed vibrational frequencies of FTIR and laser Raman spectra are

compared in Table 2.1.

Table 2.1 Comparison of vibrational frequencies from FTIR and Laser Raman spectrum of APTSC

Experimental Wavenumber (cm-1)

Band Assignments

FTIR laser Raman

2958 m) 2795 ν (C─H)

1589 (s) 1606 ν (C═N)

1497 (s) 1504 ν (C═C)

1315 1315 ν (C─NH2)

1115 1113 ρ (NH2)

1110 1010 δ (C─H) in plane

1093 - ν (C═S)

841 857 δ (C─H) out-of- plane

759 777 (s) δ (C─H)

(s) strong, (m) medium ν) stretching, (δ) bending or deformation, (ρ) rocking

2.4 NMR spectral analysis of APTSC

The proton NMR (1H) spectral analysis was carried out on the

APTSC crystal in CDCl3 using Bruker AC200-NMR Spectrometer and is

shown in Fig. 2.5. The peak observed at δ=2.31 ppm is corresponding to

methyl group (CH3). The aromatic proton ortho to imine bond is observed

at δ=7.71 ppm as a multiplet, whereas the remaining aromatic protons

are resonated at δ=7.41 ppm as a multiplet. Similarly the NH2 protons of

hydrazide part are observed at δ=8.91 ppm as a broad singlet, whereas

the NH proton is observed at δ=6.92 ppm (Silverstein and Webster 1998).

Thus the formation of hydrazone (C=N) is confirmed by NMR spectral

analysis.

25

Fig. 2.5 1H NMR spectrum of APTSC crystal

2.5 Single crystal and powder XRD studies of APTSC

The intensity data were collected at 298 K on a SADABS (Bruker,

2000) system using MoKα graphite monochromated radiation

(λ = 0.71073 Å). The molecular structure of the crystal APTSC (C9H11N3S)

was refined by the least squares method using anisotrophic thermal

parameters: R=5.1%. The crystal structure was refined by full matrix

least squares with SHELX 97 (Sheldrik 1997) program ortep drawing was

performed with ORTEP3 program (Farrujia 1999).

The compound APTSC crystallizes in orthorhombic system of Pbca.

The parameter values calculated are a=15.429(3)Å, b=7.127(7)Å,

c=8.340(7)Å, and Z=8. The molecule is nearly planar. The structure is

stabilized by N―H…S intermolecular hydrogen bonds. The molecular

structure of the APTSC together with the atom labeling scheme and the

26

intramolecular hydrogen bonding is shown in Fig. 2.6 (Santhakumari

et al., 2010). In the crystal structure pairs of intermolecular N―H…S

hydrogen bonds across a center of inversion results in the formation of

dimers generating R22(8) ring. This is a common feature observed in

similar thiosemicarbazone compounds.

Fig. 2.6 Molecular structure of the grown APTSC crystal with 50% probability ellipsoid.

The thiocarbonyl S atom acts as a single acceptor for both

hydrogen bonds. The arrangement of the non-hydrogen atoms is nearly

planar with the S and the hydrazinic NH2 group is in a transposition with

respect to the C―NH (Dario et al. 1969).

Regarding the stereochemistry of the molecule, the exocyclic angle

N1═C7―C6 [114.4(2)o] deviates significantly from the normal value of

120o because of the intermolecular non-bonded interactions between N1

and H1 of C1 with N1….H1=2.57Å, the exocyclic angles N1―N2═C8

[117.7(2)o] and N2═C8―N3 [116.0(2)o] deviate significantly from the

27

normal value of 120o. This may be due to the intramolecular (Vasuki et

al. 2002a) non-bonded interaction between N1 and H3A of N3

[N1…H3A=2.19Å]. The (C1―C6) benzene ring and the S1/N2/N3/C8

fragment are planar with the maximum deviation of 0.01(3) Å for C5 atom

from the least square plane. The widening of the exocyclic angle

N1═C7═C9 [125.8(2)o] from the normal value of 120o may be due to the

steric repulsion (Vasuki et al. 2002b and Vasuki et al. 2008) between

atoms H2A of N2 and H9A of C9 [H2A…H9A=2.15Å]. The widening of the

exocyclic angle N2―C8―S1 [123.3(2)] from 120o may be due to the non-

bonding intermolecular interactions of S1 with N2 and N3 of the

neighbouring molecules. The dihedral angle between the two planes is

34.4 (14)o. The C1―C6―C7―N1 and C5―C6―C7―N1 torsion angles are

28.4 (8)o and (146.4 (6)o) respectively. In the crystal structure, molecules

are linked by N2―H2A…S1, N3―H3B…S1 and C9―H9A…S1 hydrogen

bonds and form an infinite molecular hydrogen bond chain of APTSC

expanding along the b-axis as shown in Fig. 2.7. The recorded data are

stacked in Table 2.2.

Fig. 2.7 Projection of the crystal structure of APTSC along the b-axis. Dashed lines show the N―H…S and C―H…S interactions

28

Table 2.2 The crystal refinement data of APTSC

Crystal data Formula C9 H11 N3 S

Formula weight [g/mol] 193.27

Crystal system Orthorhombic

Crystal size [mm] 0.47 X 0.20 X 0.12

Space group Pbca

Z 8

Unit cell parameters

a [Å], b [Å], c [Å]

α [º], β [º], γ [º]

a = 15.429(3) Å, b = 5.9493(11) Å, c = 21.793(4) Å 90

Volume [Å3] 200.4(7)

F(000) 816

Data collection Radiation MoKα Wavelength [Å] 0.71073 Temperature [K] 282(2) θ min ; θ max [º] 1.87; 25.99

Range

h= -18→18, k= −7→7, l = −15→26

Refinement Refinement method Full-matrix least squares No. of parameters 1960

Final R(F) [ I>2σ (I)] reflections

R1 = 0.0539, wR2 = 0.1125 R indices (all data) R1 = 0.0838, wR2 = 0.1227

29

The powder X-ray diffraction was recorded using powder X-ray

diffractometer with CuKα radiation (λ = 1.5406 Å). Finely crushed powder

of APTSC crystal was scanned in the 2θ values ranging from 10º to 80º.

The obtained XRD peaks were indexed and is shown in Fig. 2.8. The

morphology of the grown APTSC crystal along with the indexed

crystallographic planes is shown in Fig. 2.9.

Fig. 2.8 Powder XRD pattern of APTSC

Fig. 2.9 Morphology of the APTSC crystal 2.6 High resolution X-ray diffraction analysis of APTSC

The crystalline perfection of the grown single crystal was

characterized by high resolution X-ray diffraction (HRXRD) analysis by

employing a multicrystal X-ray diffractometer (Lal and

30

Bhagavannarayana 1989) developed at National Physical Laboratory

(NPL), Delhi. The well collimated and monochromated MoKα1 beam

obtained from the three monochromator Si crystals set in dispersive

(+,-,-,+) configuration was used as the exploring X-ray beam. The

specimen crystal was aligned in the (+,-,-,+) configuration. Due to

dispersive configuration, though the lattice constant of the

monochromator crystal(s) and the specimen are different, the unwanted

dispersion broadening in the diffraction curve (DC) of the specimen

crystal is insignificant. The specimen can be rotated about the vertical

axis, which is perpendicular to the plane of diffraction, with minimum

angular interval of 0.4 arc sec. The DC was recorded by the so-called ω

scan wherein the detector was kept at the same angular position 2θB with

wide opening for its slit. Before recording the diffraction curve, the non-

crystallized solute atoms remained on the surface of the crystal and also

to ensure the surface planarity, the specimen was first lapped and

chemically etched in a non preferential etchent of water and acetone

mixture in 1:2 volume ratio. Fig. 2.10 shows the high-resolution

diffraction curve (DC) recorded for a typical APTSC single crystal

specimen using (312) diffracting plane in symmetrical Bragg geometry by

employing the multicrystal X-ray diffractometer.

The solid line (convoluted curve) is well fitted with the experimental

points represented by the filled rectangles. On disconsolation of the

diffraction curve, it is clear that the curve contains an additional peak,

which is134 arc sec away from the main peak. This additional peak

depicts an internal structural low angle (tilt angle > 1 arc min but less

than a deg.) boundary (Bhagavannarayana et al. 2005) whose tilt angle

(misorientation angle between the two crystalline regions on both sides of

the structural grain boundary ) is 134 arc sec from its adjoining region.

31

Fig. 2.10 HRXRD spectrum of APTSC

The FWHM (full width at half maximum) of the main peak and the

low angle boundary are 102 and 234 arc sec respectively. Though the

specimen contains a low angle boundary, the relatively low angular

spread of around 600 arc sec of the diffraction curve and the low FWHM

values show that the crystalline perfection is fairly good. Thermal

fluctuations or mechanical disturbances during the growth process could

be responsible for the observed low angle boundary. It may be mentioned

here that such low angle boundaries could be detected with well resolved

peaks in the diffraction curve only because of the high-resolution of the

multicrystal X-ray diffractometer used in the present study.

2.7 UV-vis-NIR spectral analysis

In order to estimate the optical transparency in the 200-1100 nm

region of the electromagnetic spectrum, the optical transmittance study

is carried out employing Varian Cary 5E UV–vis–NIR spectrophotometer

-300 -200 -100 0 100 200 3000

20

40

60

80

100

120

140

160 APTSC(312) PlanesMoKα

1

(+,-,-,+)

102"

234"

134"

Diff

ract

ed

X-r

ay

inte

nsi

ty [

c/s]

Glancing angle [arc s]

32

(Fig. 2.11). The lower cut off wavelength of the APTSC is about 300 nm

and the transmittance of the crystal is about 99% in the entire visible

and near infrared region an essential parameter required for frequency

doubling process (Rao 1984).

Fig. 2.11 UV-vis-NIR spectrum of APTSC 2.8 Z-scan technique

The Z-scan method has gained rapid acceptance by the nonlinear

optics community as a standard technique for separately determining the

nonlinear changes in refractive index and change in optical absorption

(Natarajan et al. 1996). Sheik-Bahae et al. (1989) and Van Stryland and

Sheik-Bahae (1998) have reported a single beam method for measuring

the sign and magnitude of nonlinear refractive index (n2) that has the

sensitivity compared to interferameteric methods. Using a single

Gaussian laser beam in tight focus geometry, as depicted in Fig. 2.12, the

transmittance of a nonlinear medium through a finite aperture in the far

field as a function of the sample position Z can be measured with respect

to the focal plane. The nonlinear absorption and refractive index of

APTSC crystals (thickness ~ 1.7 mm) were estimated using the single

33

beam Z-scan method with CW laser beam intensity of 35 mW and the

wavelength of 632.8 nm.

Fig. 2.12 Experimental setup for the Z-scan measurements

The study of nonlinear refraction by the Z-scan method depends on

the position (Z) of the thin samples under the investigation along a

focused Gaussian laser beam. The sample causes an additional focusing

or defocusing, depending on whether nonlinear refraction is positive or

negative. In the most reported experiments, 0.1 < S (transmittance) < 0.5

has been used for determining nonlinear refraction. Obviously, the linear

transmittance of the aperture S = 1 corresponds to the collection of all

transmitted light and therefore is insensitive to any nonlinear beam

distortion due to nonlinear refraction (Van Stryland and Sheik-Bahae

1998). Such a scheme, referred to as an “Open aperture” Z-scan, is

suited for measuring nonlinear absorption of the sample. Results

obtained from a open aperture Z-scan study for the grown APTSC are

presented in Fig. 2.13.

The nonlinear refractive index (n2) of the crystal was calculated

using the standard relations given below (Kanagasekaran et al. 2008).

(2.1)

where ΔTP-V is the difference between the normalized peak and valley

transmittance. ΔTP-V / Δφo is the on-axis phase shift at the focus. The

ΔTP-V = 0. 406 (1 - S)0.25 | Δφo |

34

nonlinear refractive index (n2) and nonlinear absorption coefficient (β) are

given by

02

o eff

nkI L

Δφ= (2.2)

Δβ =

o eff

2 2 TI L

(2.3)

where k is the wavenumber (k=2/λ) and Leff = [1–exp(-αL)]/α with

Io = P/(ωo2) defined as the peak intensity within the sample, where L is

the thickness of the sample and α is the linear absorption coefficient. The

ratio of the signals with and without the aperture accounts for the

nonlinear absorption and gives the information about purely nonlinear

refraction. The enhanced transmission near the focus is indicative of the

saturation of absorption at high intensity. Absorption saturation in the

sample enhances the peak and decreases the valley in the closed

aperture Z-scan. The focusing effect is attributed to a thermal

nonlinearity resulting from absorption of radiation at 632.8 nm. Localized

absorption of a tightly focused beam propagating through an absorbing

medium produces a spatial distribution of temperature in the crystal and

consequently, a spatial variation of the refractive index that act as a

thermal lens resulting in phase distortion of the propagating beam (Fig.

2.12). The nonlinear absorption property of the D--A type -electron

system can be related closely to the -electron conjugate degree and

delocalization capacity of the molecule. The three dimensional X-ray

crystal structure solution of this crystal showed that the torsion angles

C1―C6―C7―N1 and C5―C6―C7―N1, in these conjugated chains are

coplanar (Fig. 2.6). They are all favorable to nonlinear optical absorption,

especially to saturated absorption. The calculated value of nonlinear

refractive index (n2) of the APTSC crystal is 5.11 x 10-8 cm2/W and the

35

nonlinear absorption coefficient (β) estimated from the open Z-scan curve

is 0.1763 cm/W.

Fig. 2.13 Z-scan open spectrum of APTSC

2.9 Fluorescence studies of APTSC

Fluorescence may be expected generally in molecules that are

aromatic or contain multiple conjugated double bonds with a high degree

of resonance stability (Willard et al. 1986). Fluorescence finds wide

application in the branches of biochemical, medical and chemical

research fields for analyzing organic compounds. The excitation and

emission spectra for APTSC were recorded using Varian Carry Eclipse,

Fluorescence Spectrometer. The excitation wavelength was varied in the

range 200-400 nm (Fig. 2.14) and the sample was excited at 239 nm. The

emission spectrum (Fig. 2.15) was measured in the range 300-600 nm

and the emission peak was observed about 388 nm. Thus the results

indicate that APTSC crystal has the blue fluorescence emission.

36

Fig. 2.14 Excitation spectrum of APTSC

Fig. 2.15 Emission spectrum of APTSC

250 375 500 625 750

0

10

20

30

40

50

Wavelength (nm)

Inte

nsit

y (

A.U

)

200 240 280 320 360

0

50

100

150

200

Wavelength (nm)

Inte

ns

ity

(A

.U)

37

2.10 Thermal analysis of APTSC

Thermogram provides information about decomposition patterns of

materials and weight loss (Earnest 1984). The thermal stability of APTSC

was studied by thermogravimetric analysis (TGA) and differential thermal

analysis (DTA) using SDT Q600 V8.3 Build 101 instrument between the

temperatures 50 °C and 1100 °C at a heating rate of 10 °C /min under

Nitrogen atmosphere (Fig. 2.16). The APTSC sample weighing 3.6400 mg

is taken for the measurement. The major weight losses observed about

220 °C and 450 °C are due to the decomposition of APTSC crystal. From

the DTA curve it is observed that, the material is stable up to 165 °C , the

melting point of the substance.

Above this point, the material begins to attain an endothermic

transition and begins to decompose (Willard et al. 1986) The sharpness of

this endothermic peak shows a good degree of crystallinity of the sample

(Fur et al. 1995).

Fig. 2.16 TGA/DTA curve of APTSC crystal

38

2.11 Microhardness analysis of APTSC

Vickers microhardness test was carried out on APTSC using

microhardness tester fitted with a diamond indenter. The indentations

were made using a Vickers pyramidal indenter for the loads 25 g, 50 g

and 100 g. The diagonals of the impressions were measured using

Shimadzu (Japan), Model HMV-2 hardness instrument. The

measurements were made on the well-developed face. The indentation

time was kept as 25 sec for all the loads. Vickers microhardness number

(Hv) was evaluated from the relation Hv = 1.8544(P/d2 ) Kg/mm2, where P

is the applied load in g and d is the diagonal length of the impression in

mm. The variation of microhardness values with applied load is shown in

Fig. 2.17. From the Vicker’s microhardness studies, it is observed that

the hardness value increases upto a load of 100 g. For the load above

100 g crack started developing around the indentation mark which may

be due to the release of internal stresses (Ushasree et al. 1998). The

presence of cracks confirms the decrease in microhardness (Li and Bradt

1991).

Fig. 2.17 Microhardness values vs load for APTSC crystal

39

Work hardening coefficient n, a measure of the strength of the

crystal is computed from the log P-log d plot (Fig. 2.18 ) and it is found to

be 3.01. Onitsch (1950) inferred that the value of n lies between 1 and

1.6 for hard materials and for soft materials it is above 1.6. Thus the

APTSC crystal comes under the soft material category.

Fig. 2.18 The plot of log P vs log d of APTSC

40

Section B

Growth and characterization of benzaldehyde thiosemicarbazone

monohydrate (BTSC) single crystals

2.12 Experimental

2.12.1 Synthesis, solubility and growth of BTSC single crystals

The BTSC was synthesized following the procedure given by

(Sheng-Jiu Gu and Kai-Mei Zhu 2008) by reacting benazaldehyde and

thiosemicarbazide in 100 ml flask in the presence of aqueous medium.

After refluxing it for 2 hrs at 100 °C the mixture was allowed to cool

slowly to room temperature which yeilded colourless crystalline powder

solid of the compound. The reaction is depicted in Scheme. Repeated

recrystallization of BTSC from ethanol was carried out to improve the

purity of the compound. As a first step towards crystallization, the

selection of suitable solvent is very definitive for the growth of good

quality single crystals (Sherwood 1998 and Holden and Singer 1960).

Scheme. The reaction mechanism of BTSC

The solubility of BTSC was determined at five different

temperatures, viz, 25, 30, 35, 40 and 45 °C. The solubility at 25 °C was

determined by dissolving the BTSC salt in 100 cc ethanol taken in an air-

tight container and placed in a water bath with continuous stirring. After

attaining the saturation the concentration of the solute was estimated

gravimetrically. The same procedure was repeated to estimate the

41

solubility at different temperatures (Balakrishnan and Ramamurthi 2007)

and the results are presented in Fig. 2.19.

Fig. 2.19 Solubility curve of BTSC

The purified colorless crystalline sample of BTSC was dissolved

thoroughly in ethanol at 32 °C to form the saturated solution. The pH of

the solution was about 4. The saturated solution of 50 ml was taken in a

beaker and was properly sealed and placed in a constant temperature

bath. The solvent was allowed to evaporate slowly at room temperature.

Well defined crystals with good transparency appeared in the beaker in a

growth period of 10 days.

One of the best crystals was selected as the seed crystal and

inserted into a 100 ml beaker containing the solution of pH~4 and placed

in a constant temperature bath maintained at 30 °C. Single crystal of size

10 x 10 x 3 mm3 was harvested by slow solvent evaporation method in a

growth period of 25 days. The as-grown crystal is shown in Fig. 2.20.

42

Fig. 2.20 The as-grown BTSC crystal

2.13 Single crystal X-ray diffraction studies of BTSC

The X-ray diffraction (XRD) data were collected using a computer-

controlled Enraf Nonius-CAD 4 single crystal X-ray diffractometer. Single

crystal of suitable size was selected for the X-ray diffraction analysis and

the unit cell parameters were determined using 25 reflections. XRD

results show that BTSC crystal belongs to the orthorhombic system and

the unit cell parameters obtained are in great agreement with the

reported values of Jiu Gu and Kai-Mei Zhu (2008) (Table 2.3). It has

been observed that BTSC crystal belongs to the most popular space

group of P212121. The crystal structure of BTSC is reported to be

stabilized by N―H…N intramolecular hydrogen bonds and contributes to

the molecular conformation. In addition water molecules are involved in

the intermolecular N―H…O and O―H…S hydrogen bonds, which link the

molecules into ribbons extended along the a-axis. Weak intermolecular

N―H…S hydrogen bonds link these ribbons into layers parallel to the ab

plane with phenyl rings pointing up and down (Sheng Jiu Gu and Kai-

Mei Zhu 2008).

43

Table 2.3 Unit cell parameters of BTSC

2.14 FTIR spectral analysis of BTSC

Fourier transform infrared (FTIR) spectrum of BTSC was recorded

using Perkin Paragon-500 by KBr pellet technique between 400 cm-1 and

4000 cm-1 and is shown in Fig. 2.21. Intense sharp peak was observed at

3241 cm-1 due to O―H stretching vibration of H2O. Since the stretching

vibrations of the OH bonds in water molecules and the deformation

vibrations of the water molecules fall into the region of the deformation

vibration of NH2 group, it is difficult to determine the presence or absence

of water molecules on the basis of the FTIR spectrum alone.

Fig. 2.21 FTIR spectrum of BTSC crystal

Cell parameters Present work Sheng Jiu Gu and Kai-Mei Zhu (2008)

a (Ǻ) 6.240(2) 6.1685(10)

b (Ǻ) 7.5820(10) 7.6733(12)

c (Ǻ) 21.129(36) 21.131(2)

Volume((Ǻ3) 998.3(9) 1000.2(2)

System Orthorohombic P212121

44

As expected, the peak corresponding to imine group (C═N) was

observed at 1595 cm-1, which confirms the formation of the imine bond

between aldehyde and hydrazide. The peaks lying below 1500 cm-1 could

be due to C═N and N―N stretching vibration. As it is very broad, nearly

all NH2 groups in the crystal are expected to be in hydrogen bonding

interaction with neighbouring groups. The C―H stretching absorption, a

weak absorption, is observed at 3026 cm-1 (Silverstein and Webster 1998

and Sankar et al. 2007). The C═C stretching (medium) absorption is

confirmed from the band at 1534 cm-1. The deformation vibrations of

NH2 group is confirmed from the band at 1365 cm-1. The C═S stretch of

thiosemicarbazide moiety is observed at 1100 cm-1.

Absence of characteristic aldehyde band at 2720 cm-1 indicates that

there is no aldehyde group in the final product (Willard et al. 1986). NH2

asymmetric stretching is confirmed at 3156 cm-1. The peak at 686 cm-1 is

due to the N―H out-of-plane bending. As mono substituted benzenes

show C―H deformation in the region 700-900 cm-1, the peaks at 868 cm-1

and 754 cm-1 are due to C―H deformations. Thus the FTIR spectrum

confirms the formation of BTSC.

2.15 Linear and nonlinear optical properties

In order to estimate the optical transparency in the 200-1100 nm

region of the electromagnetic spectrum, the optical transmittance study

is carried out on the BTSC crystal of thickness ~2 mm employing Varian

Cary 5E UV–vis–NIR spectrophotometer (Fig. 2.22). The cutoff wavelength

of the BTSC is observed ~370 nm and the crystal is found to be

transparent in the region of ~400-1100 nm. The hump at 220 nm is due

to the electronic excitation of the aromatic compound containing sulfur

and nitrogen (Chatwal 2004).

45

Fig. 2.22 UV-vis-NIR spectrum of BTSC

The optical absorption coefficient (α) was calculated using the following

relation

α = 2.303 log (1/T) (2.4)

d

where T is the transmittance and d is the thickness of the crystal. The

various other optical constants were calculated using the following

theoretical formulae (Heinkhen 2008, Pankove 1971).

The extinction coefficient is obtained in terms of the absorption

coefficient,

K = λα/4π (2.5)

The reflectance in terms of absorption coefficient is derived as,

√1± (1-exp(-αd)+exp(αd)

R = (2.6) 1+exp(-αd)

and the linear refractive index is given by the relation

46

-(R+1) ± √(-3R +10R-3) (2.7)

n = 2(R-1)

The complex dielectric constant is related to the refractive index

and the extinction coefficient as

εc = εr + εi (2.8)

where the real (εr) and imaginary (εi) dielectric constant are given as

εr = n2 – K2 (2.9)

εi = 2nK (2.10)

The optical conductivity (σop ) is a measure of the frequency response of

the material when it is irradiated with light

σop = αnc/4π (2.11)

where c is the velocity of light. The electrical conductivity can also be

estimated by optical method using the relation

σe = 2λ σop / α (2.12)

The Z-scan is a simple and popular experimental technique to

measure the intensity dependent third order nonlinear susceptibility of

the materials. It allows the simultaneous measurement of both the

nonlinear refractive index and the nonlinear absorption coefficient. In

this method, the sample is translated in the Z-direction along the axis of

a focused Gaussian beam from the He-Ne laser at 632.8 nm and the far

field intensity is measured as a function of the sample position. The

schematic diagram of Z-scan technique is shown in Fig. 2.12. By properly

monitoring the transmittance change through a small aperture at the far

field position (closed aperture), one is able to determine the amplitude of

the phase shift. By moving the sample through the focus and without

47

placing an aperture at the detector (open aperture) one can measure the

intensity dependent absorption of the sample.

When both the methods (open and closed) are used for the

measurements, the ratio of the signals determines the nonlinear

refraction of the sample. The energy dependence of the absorption

coefficient suggests the occurrence of direct band gap and hence it obeys

the relation for high photon energy, (α�ν)2 = A(hν-Eg), where Eg is the

optical band gap and A is a constant. The variation of (α�ν)2 versus hν in

the fundamental absorption region is pictured in Fig. 2.23 and Eg

evaluated by the extrapolation of the linear part to the x-axis is 3.6 eV.

Fig. 2.23 (αhν)2 vs hν

From the recorded absorption spectra, various linear optical

constants of BTSC were calculated and the variation of these optical

constants as a function of photon energy is plotted (Figs. 2.24, 2.25). The

refractive index of the material at 211 nm is 1.332. Also the extinction

coefficient shows exponential decay as the photon energy increases.

Refractive index being the measure of percentage of intensity of light

reflected, the reflectance shows an increasing value along the photon

energy.

(αhν)2x106 x (eV/m)2

48

Fig. 2.24 Extinction coefficient as a function of photon energy

Fig. 2.25 Reflectance as a function of photon energy

49

From Figs. 2.24 and 2.25, it is clear that the extinction coefficient

and the reflectance depend upon the absorption coefficient. The internal

energy of the device depends on this absorption coefficient. The high

transmission and low reflectance of BTSC in the UV-Vis-NIR region reveal

that the material may be used in antireflection coating in solar thermal

devices and nonlinear optical applications. The low extinction value (10-3)

shows the semiconducting nature of the material (Pankove 1971). This

makes the material more prominent for device applications in information

processing and computing. The third order nonlinear refractive index and

the nonlinear absorption coefficient were evaluated by the Z-scan

measurements using the equations 2.1-2.3 described in section-A

(Chapter 2) and the results are shown in Figs. 2.26a and 2.26b

respectively.

Fig. 2.26a Z-scan open aperture

Open Curve

0

0.5

1

1.5

2

2.5

3

3.5

-15 -10 -5 0 5 10 15

Z-Position (mm)

Nor

mal

ised

Tra

nsm

itta

nce

50

Fig. 2.26b Z-scan closed aperture

The real and imaginary parts of the third order nonlinear optical

susceptibility χ(3) are defined by (Heinkhen, 2008, Pankove, 1971)

Re (χ(3)) (esu) = 10-4 (ε0c2no2n2) / (cm2/W) (2.13)

Im (χ(3)) (esu) = 10-2 (ε0c2no2n2) / 42 (cm/W) (2.14)

where ε0 is the vacuum permittivity, no is the linear refractive index of

the sample and c is the velocity of light in vacuum. Table 2.4 portrays the

experimental details and the results of the Z-scan technique for BTSC

crystal. The calculated value of the nonlinear refractive index n2 is

−3.126 x 10-8 cm2/W.

From the open aperture Z-scan curve (Fig. 2.26a) it can be

concluded that as the minimum lies near the focus (Z=0), the nonlinear

absorption is regarded as two photon absorption. The nonlinear

Closed curve

0

1

2

3

4

5

6

-10 -5 0 5 10

Z-Position in mm

No

rma

lis

ed

Tra

ns

mit

tan

ce

51

absorption coefficient is found to be 4.076 x 10-3 cm/W. The third order

susceptibility of BTSC is 3.915 x 10-6 esu.

Table 2.4 Meaurement details and the results of the Z-scan

technique

2.16 Thermal analysis

Thermogram provides information about the thermal properties of

materials (Earnest 1984). The thermal stability of BTSC crystal was

studied by thermogravimetric analysis (TGA) and differential thermal

analysis (DTA) using SDT Q600 V8.3 Build 101 instrument between the

temperatures 50 °C and 1100 °C at a heating rate of 10 °C /min in

nitrogen atmosphere (Fig. 2.27). The BTSC sample weighing 1.745 mg

was taken for the measurement.

Laser beam wavelength 632.8 nm

Focal length of the lens 24 cm

Optical path length 175 cm

Beam radius of the aperture (ωa) 4 mm

Aperture radius (ra) 4 mm

Sample thickness (l) 1.7 mm

Beam radius (ωL) 3 mm

Effective thickness (Leff) 1.69 mm

Linear absorption efficient 0.625

Nonlinear refractive index(n2) −3.126 x 10-8 cm2/W

Nonlinear absorption coefficient (β) 4.076 x 10-3 cm/W

Real part of the third-order susceptibility [Re ( X 3 )]

3. 27 x 10-6 esu

Imaginary part of the third-order susceptibility [Im ( X 3 )]

2.15 x 10-6 esu

Third-order susceptibility ( X 3) 3.915 x 10-6 esu

52

There are three weight losses noted in the thermogram. The earlier

one is due to expulsion of water present in the crystal. The second and

third major weight loss is observed just above 220 °C and 400 °C. It is

due to decomposition of BTSC crystal. From the DTA curve it is observed

that, the material is stable up to 153 °C, the melting point of the

substance. The melting point, also measured directly using a GUNA

melting point apparatus, confirmed this value. Above this point, the

material begins to attain an endothermic transition and begins to

decompose (Willard et al 1986).

Fig. 2.27 TGA/DTA curves of BTSC

2.17 Nonlinear optical studies

Kurtz and Perry (1968) second harmonic generation test was

performed to estimate the NLO efficiency of powdered BTSC crystal. The

grown single crystal of BTSC was powdered with a uniform particle size

and then packed in a micro capillary of uniform bore and was illuminated

53

using Spectra Physics Quanta Ray DHS2: Nd:YAG laser using the first

harmonics output of 1064 nm with pulse width of 8 ns and repetition rate

10 Hz. The second harmonics signal, generated in the crystal was

confirmed from the emission of green radiation by the crystal. A sample

of potassium dihydrogen orthophosphate, also powdered to the same

particle size as the experimental sample, was used as a reference

material in the present measurement. The SHG radiations of 532 nm

green light was collected by a photomultiplier tube (PMT-Philips

Photonics-model 8563) after being monochromated (monochromator-

model Triax-550) to collect only the 532 nm radiation. The optical signal

incident on the PMT was converted into voltage output at the CRO

(Tektronix-TDS 3052B). The input laser energy incident on the powdered

sample was chosen to be 3.4 mJ. Thus the Powder SHG efficiency

obtained for BTSC is about 5.3 times that of potassium dihydrogen

orthophosphate crystal. This may be attributed to the existence of

intermolecular N―H…N and N―H…S hydrogen bonds and N―H…O and

O―H…S hydrogen bonds due to water molecules in BTSC, which link the

molecules into ribbons extended along the a-axis (Jiu Gu and Kai-Mei

Zhu 2008).

2.18 Microhardness studies

Measurement of hardness is a useful nondestructive testing

method to determine the hardness of the materials. The micro hardness

value correlates with other mechanical properties such as elastic

constants and yield strength. Vickers microhardness test was carried out

on BTSC single crystal using Vickers hardness tester fitted with

pyramidal indenter. Several trials of indentation (Mott 1956, Wyatt and

Hughes 1974, Tabor 1951, Neill 1967 and Smith and Sandland 1923)

were carried out on the prominent (0 1 1) face of the crystal and the

average diagonal length was calculated for an indentation time of 15 sec.

The Vickers hardness number is calculated using the relation

54

22

PH 1.8544 kg / mm

dν = (2.15)

where P is the applied load in kg and d is the average diagonal length of

impression in mm. The variation of Hv with applied load for BTSC crystal

is shown in Fig. 2.28. Cracks were observed for loads more than 100 g.

From the Vicker’s microhardness studies, it is observed that the

hardness value increases upto a load of 100 g. Cracks developed around

the indentation mark for loads above 100 g. The formation of cracks

confirms the decerease in microhardness (Hays and Kendall 1973). Work

hardening coefficient n, a measure of the strength of the crystal, is

computed from the plot of log P vs log d.

The plot of log P vs log d of BTSC crystal yields a straight line and

its slope, the work hardening index n, is found to be 2.08.

Fig. 2.28 The variation of hardness number vs load P

According to Meyer’s law ( Meyer 1951, Gong 2005) ,

20 40 60 80 10072

74

76

78

80

82

84

86

Har

dn

ess

nu

mb

er (

Kg

/mm

2 )

Load P (g)

55

(2.16)

where K1 is the standard hardness found out from the P versus dn graph.

It is known that the material takes some time to revert to elastic mode

after the applied load is removed, so a correction x is applied to the

observed d value. Kick’s law may be modified as,

(2.17)

Simplifying eqns. (2.16) and (2.17) become

(2.18)

The slope of dn/2 verses d yields (K2/K1)1/2 and the intercept is a measure

of x.

The fracture toughness (Kc) is given by

Kc = P / (β × C3/2) (2.19)

where C is the crack length measured from the centre of the indentation

mark to the crack tip, P is the applied load and geometrical constant

β = 7 for Vicker’s indenter. The brittleness index (B) is given by

B = Hv / Kc (2.20)

The yield strength (σv) of the material can be derived using the

relation

}])n()n(.

[)]n({[.

H nVv

2

21251221

92−

−−−×−−=σ (2.21)

Onitsch (1950) inferred that for hard materials the value of n lies

between 1 and 1.6 and for soft materials it is above 1.6. Thus the BTSC

crystal comes under the soft materials category. The load dependent

ndKP 1

=

22 )( x

dK P + =

xKKdK Kd n 2/112

2/ 1 1 2

2/ )/()/ ( + =

56

hardness parameters n, K1, K2, fracture toughness (Kc), brittleness index

(B) and yield strength (σv) were calculated for the BTSC crystal and are

given in Table 2.5.

Table 2.5 Micro hardness value obtained on the BTSC crystal

Parameters Values

n 2.08

K1 (Kg/mm) 27.7

K2 (Kg/mm) 30.16

Kc (g/μm3/2) 0.0535

Bi (m-1/2) 1.58 x 108

σy (MPa) 216.8